Earth microbes may survive on Mars

作者：步矾氘 发布时间：2019-03-02 06:02:02

By Maggie McKee Terrestrial microbes that hitch a ride to Mars on spacecraft may be able to survive under special circumstances, according to a new laboratory study. The research suggests scientists should take extra care when analysing potential signs of life during future missions to the Red Planet. Most spacecraft that touch down on Mars have not been thoroughly sterilised by heat or radioactivity – so they carry with them living microbes from Earth. But Mars’s thin atmosphere allows such intense ultraviolet radiation to reach the planet’s surface – triple that found on Earth – that any life inadvertently carried on the spacecraft is thought to be wiped out quickly. Indeed, Martian-level doses of UV radiation have destroyed some microbe species in just seconds in laboratory tests. But now, an international team has tested the endurance of a particularly hardy type of blue-green alga – or cyanobacterium – that thrives in dry deserts from Antarctica to Israel. The resilient bacterium, called Chroococcidiopsis sp. 029, was chosen as a “worst-case scenario” for contamination of the planet. The team found that dormant spores of the bacterium had mostly died after five minutes of Martian UV exposure. However, the bacteria were able to stay alive if they were shielded by just 1 millimetre of soil during the tests, which ran for up to 24 hours. Under such a protective coating, the bacteria “could survive – and potentially grow – under the high Martian UV flux if water and nutrient requirements for growth were met”, write the researchers in the journal Astrobiology. “We think there are places on Mars where Earth life could make a living,” says John Rummel, NASA’s planetary protection officer in Washington, DC, who is charged with preventing microbes from contaminating worlds beyond Earth. He says this study shows “even the toughest stuff doesn’t survive for long” on the surface of spacecraft, but he says live microbes probably do take shelter within the spacecraft bodies. So if a Mars rover fell off a cliff or a spacecraft broke open in a crash – like NASA’s lost Mars Polar Lander may have – these microbes might find a toehold for survival provided they landed in the right place, he says. Such places include Martian gullies that may periodically be flooded with liquid water, or areas around the poles, where “microbes and ice could make a happy partnership”, Rummel told New Scientist. Another finding in the study may also help shape future missions to Mars, says Andrew Schuerger, one of the research team. He is a plant pathologist at the University of Florida in Gainesville and conducts research at NASA’s Kennedy Space Center. The study revealed that the “biogenic” signs of life, including the molecular signatures of DNA and chlorophyll, are detectable for hours – and perhaps much longer – after the bacteria themselves have died. So Schuerger says future lander missions to Mars should contain experiments to detect both life itself – in the form of live cells – as well as these signature molecules. “I think it was common to believe that finding one or the other was enough to say there was life,” Schuerger told New Scientist. “But you have to look for both simultaneously to be sure you’re detecting life on Mars – or life from Earth that made the trip to Mars.” More on these topics: